1. Field of the Invention
The invention in general relates to navigation systems, and more particularly to an acoustic system for determining bearing and range to objects via passive sensors.
2. Background
A variety of methods and systems for detecting and tracking objects using acoustic information are well known in the art. These systems include both active and passive approaches. While active systems allow for rapid determination of both range and bearing to objects, passive systems can be more desirable where, for example, there are concerns about remaining undetected (e.g., military applications) or in minimizing the acoustic energy being generated (e.g., when in proximity to sea animals sensitive to sonar).
Most prior art passive systems have employed approaches that use the acoustic energy radiated from an object to determine its position. This process may involve extra processing or maneuvering, as in the case of target motion analysis (see, e.g., U.S. Pat. No. 5,216,815 to Bessacini) or Ekelund ranging (see, e.g., U.S. Pat. No. 5,877,998 to Aidala et al.). Additional information such as frequency parameters of the radiating object have also been considered (see, e.g., U.S. Pat. No. 5,432,753 to Maranda). However, in all these cases it is the acoustic energy from the object of interest that is measured, and the energy from other objects is either ignored or filtered.
A different approach for passive detection was suggested by John Potter in his article, xe2x80x9cAcoustic Imaging Using Ambient Noise.xe2x80x9d This article describes the results of a computer simulation of modeling reflectors (e.g., a spherical shape) scattering ambient acoustic energy (e.g., isotropic and partial anisotropic noise). Its conclusions were that near-perfect reflectors in isotropic noise cannot be imaged, except in the near-field, but that in typical oceanic environments there should be considerable anisotropy such that imaging is possible. In conducting their simulation several simplifying assumptions were made regarding the model for scattered pressure amplitude of a source wave, and the ranges were taken as givens. In other words, while this article pointed to the possibility of applications in which scattering objects might be imaged, it did not describe a working solution for how to carry out that imaging. Epifanio et al., and Potter and Chitre more recently improved on this by showing how images could be formed via intensity mapping of a region at various frequencies. However, the practical range for their acoustic imaging was limited; Epifanio et al. imaging fixed objects between 13 m and 38 m. Thus, to date the systems based on this acoustic imaging concept only address short range, near-field imaging applications, and have not attempted to address issues like range determination.
One embodiment of the invention includes a multistatic time-aligned source combiner, that exploits the cross-correlation between a target beam and all other beams to detect and measure the range of the target. This combiner performs long-term cross-correlation between the target beam and each individual source beam, and then aggregates the individual correlation results, time aligned based on the scenario geometry, and averaged to enhance the composite signal-to-noise ratio of the range measurement. In doing so, this embodiment exploits innocuous acoustic sources opportunistically as if the array were part of a multi-static, active (e.g., sonar) system.
A partial listing of benefits that may be realized from this embodiment include: high-resolution target location can be obtained using passive acoustic listening arrays; target ranges may be determined without emitting sounds that might compromise the presence or location host vessel or injure marine life; conventional signal detection methods may be enhanced synergistically with time-aligned source combining; and diverse acoustic sources, including discrete/opportunistic and natural/distributed sources, may be exploited in determining location information.